An experimental study investigating the interaction of linguistic complexity and performance in child language acquisition tests the hypothesis that children learning a first language acquire relatively complex sentences somewhat later than less complex sentences. In one of three tests, the subjects, 44 children aged 3.6 to 6 years, were presented with a series of structurally diverse sentences about toys within reach. The children's comprehension of the sentences was observed in their reactions as measured by their movements of the toys. The second and third tests used two imitation tasks focusing on adverbial structures, using the same toy-moving sentences, varying in length, and toy-moving sentences carefully controlled for length, to find the subjects' critical sentence length for imitation. Analysis of the various responses to sentence types and complexity in all three tests supported the "weak complexity" hypothesis and suggest that the linguistic factors affecting performance are task-dependent and deny a simple one-to-one relationship between complexity and performance. (MSE)